LePaul,
You are maxing at those frame rates because you have vertical sync turned on. Vertical synchronization syncs your refresh rates to the monitor's electron gun vertical scanning speed (in your case 75Hz). The electron guns in a monitor scan in a raster, line-by-line from the top left to right and down the screen. Once it reaches the bottom, it jumps back up to the top left and starts over. Vertical sync synchronizes your frame rate to that scan speed. 75Hz means your monitor draws a new screen 75 times per second.
If you turn vert sync off, your frame rates will go to the max your video card can push. The downside to that is your video will not be quite as smooth and under high rates of turn or roll, you may see some slight jitter. Turning vsync off does nothing except provide bragging rights as to how high your frame rates can get...and with a gf4 ti4600, it can get pretty darn high (I often see 280+ fps with mine). Although this does nothing for game performance, it is a very general indication of the power of your video card and can give you an idea of how much strain it will handle without losing too many frames per second.
On the plus side, there really is no advantage to having the vert sync off. Any frame rate over 30 is a waste. What you need to be concerned with is how low your frame rates get when the graphics get really heavy (that's where you are really straining the video card). Locking refresh rate to scan rate with vsync on limits ONLY the high frame rates. Your video card will perform the same when the graphics are very heavy. Again, as long as you see at least 30 fps, you are good to go.
